3 Answers
3

I saw the following ideas in The Encyclopedia of Talmudic Disputes and Perspectives: R. Yochanan and Resh Lakish by R. Nachman Cohen, which attempts to apply a few "big ideas" which divide R"Y and R"L as the reasons behind their many disagreements:

Resh Lakish holds that the Yetzer Hara is a creation unto itself and enters the human causing him to sin. The source is:

Resh Lakish said: A person does not
transgress a sin unless he is invaded
by a spirit of insanity (R. Cohens
traslation- Demon of Frivolity)...
(Sotah 3a)

The idea of an outside force causing sin is in line with another statement made by Resh Lakish:

The Satan, the Yetzer Hara and the
Angel of Death are the same... (Bava
Basra 16a)

R. Cohen posits that not everyone agrees with the idea of an outside force causing sin, but rather that the "Yetzer Hara" and Yetzer Tov" are manifestations of the human condition. While he brings many chachamim who agree to this (R' Avraham ben haRambam, Rabeinu Bachye and R' Yisrael Salanter), his sole Talmudic source is a Midrash Tanchuma (Breishis 7):

...If you say 'why did He create the
Yetzer Hara...is one (now) able to do
good?' Hashem said 'You make it bad!
Why does a child of 5, 6, 7, 8, and 9
not sin; rather from 10 and on he
empowers his Yetzer Hara'...

The Midrash continues to blame a persons inability to control his Yetzer hara on his apathy.

R. Cohen makes an assumption that R. Yochanan holds of the latter approach. He applies these different approaches to other differences between R"L and R'Y. For example, R"L held that a wicked person who repents is greater than he was before sinning, because his pure soul reached a higher level through love of G-d (Tanchuma- R"L on Yechezkel 18:27), whereas R"Y held that a soul that sins is no longer pure, but internally damaged and cannot return to its former state even through repentance (based on Breishis Rabba 21:8).

Summary: Some hold that the Yetzer haRa is an external force. Others hold it is a manifestation of the human condition.

Very important philosophical dichotomy - internal yetzer vs. external yetzer. This goes a long way in answering its relationship to satan as well (according to Resh Lakish), which must be external.
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WAFJun 13 '11 at 14:46

Rav Hirsch explains that it is the temptation toward the physical. As a result of this definition it is not categorically bad (nor is the yetzer tov necessarily good). Only the majority of the time does it lead people to negative/destructive things. However, when it overtook the yetzer tov, that was enough to seal the world's fate for annihilation by flood.